The invention relates to manufacturing of superconductive Nb.sub.3 Sn layers on niobium surfaces for high frequency applications in general and more particularly to an improved method of manufacturing such layers by diffusing tin into the niobium surface at elevated temperature.
Superconductive devices for operation with electromagnetic high frequency fields, the frequencies of which extend to over 10 GHz, can be used as resonators and separators for particle accelerators and also as high frequency resonators for other purposes, e.g., as frequency standards. They can be designed for this purpose particularly as cavity resonators or resonator helices. Superconducting cavity resonators are generally operated in a frequency range of between 1 and 15 GHz, and superconducting resonator helices in the range around 100 MHz. Niobium and, on occasion, lead are provided as superconductor materials for such resonators.
In such superconducting devices, one strives for a high quality factor Q and also generally for a critical magnetic flux density B.sub.c.sup.ac, measured in the presence of high frequency fields as high as possible, so that the superconducting devices can be operated with a high frequency power as large as possible and at the same time with low surface resistance. For, if the critical magnetic flux density B.sub.c.sup.ac is exceeded, the losses increase steeply, the surface resistance is increased considerably and the electromagnetic field breaks down. An upper limit for the critical magnetic flux density B.sub.c.sup.ac in this connection is what is known as the dynamic critical flux density B.sub.c. Since this flux density is higher for Nb.sub.3 Sn than for pure niobium, a higher critical flux density B.sub.c.sup.ac can be expected at an Nb.sub.3 Sn surface than at a niobium surface. In addition, Nb.sub.3 Sn also has a considerably higher critical temperature than niobium, so that its thermal stability is correspondingly higher. Nb.sub.3 Sn is therefore suitable for higher operating temperatures than niobium. With Nb.sub.3 Sn surfaces, operation at the temperature of boiling liquid helium, about 4.2 K, in particular, is therefore possible, while corresponding niobium surfaces must be operated at substantially lower temperatures due to their high frequency losses.
Therefore, thin layers of Nb.sub.3 Sn have been applied on niobium resonators by first vapor depositing tin on the niobium resonator and then subjecting the latter to a heat treatment. With such surface layers, a Q.sub.o factor of about 10.sup.9 at 2.8 GHz and a critical magnetic flux density B.sub.c.sup.ac of about 25 mT can be obtained (cf. "Siemens-Forschungs- und Entwicklungsberichte" (Research and Development Reports), vol. 3 (1974), page 96).
In such a method, the difficulty arises, however, that the vapor deposited tin melts at the beginning of the heat treatment and can easily run, for instance, in the case of inside coating of cavity resonators, along the inside surface to the lowest point of the cavity, before enough tin for forming an Nb.sub.3 Sn layer of sufficient thickness diffuses into the niobium surface. In practice, only very thin tin layers can therefore be vapor deposited and the vapor deposition and subsequent heat treatment must be repeated several times so that a sufficient amount of tin can diffuse into the niobium surface to form the Nb.sub.3 Sn layer.
It is further known to expose the niobium parts to be provided with an Nb.sub.3 Sn layer to a tin vapor atmosphere in a closed reaction vessel, e.g., a sealed, evacuated quartz ampoule, at an elevated temperature of about 1000.degree. C. The tin diffuses from the tin vapor atmosphere into the surface, forming the desired Nb.sub.3 Sn layer. With this method, Nb.sub.3 Sn layers several micrometers thick with Q.sub.o factors of about 10.sup.9 and critical magnetic flux densities of more than 40 mT can be obtained at 1.5 K ("IEEE Transactions on Magnetics", vol. MAG-11, No. 2, March 1975, pages 420 to 422). However, the sealed reaction vessel of this device must generally be destroyed in the opening after the coating. Since, for coating larger niobium parts, correspondingly large vessels, e.g., ampoules, are required, the known method is accordingly expensive. In addition, the gases produced in or after the sealing off remain enclosed in the reaction zone if one works with a sealed reaction vessel. These gases can lead to disturbances of the Nb.sub.3 Sn layer. However, the quality of this Nb.sub.3 Sn surface layer is of decisive importance, since the depth of penetration of the high frequency currents and fields into the superconductor surface is only about 0.1 to 0.2 .mu.m.
To avoid these difficulties which arise in the case of sealed reaction vessels, open reaction chambers may also be provided, in which gases present or generated within the reaction zone can be drawn off. Here, the reaction zone must be sealed off to the extent that a tin vapor pressure sufficient for forming the Nb.sub.3 Sn layer in a relatively short time is maintained even though the reaction chamber is open, and an excessive amount of tin is prevented from diffusing away.
In the last mentioned cited methods, in which an Nb.sub.3 Sn layer is formed by exposing the niobium parts to a tin vapor atmosphere at an elevated temperature of about 1000.degree. C., there is the danger, however, that the niobium surface parts will only be coated nonuniformly. Thus, places with often finely distributed spotty shapes are observed on the completely coated surface portions, for instance, on which there are no, or only substantially thinner Nb.sub.3 Sn layers. These disturbances can often be diminished by expensive additional measures such as, for instance, by pre-anodizing the surface portions in conjunction with a temperature lead of a tin source with respect to these surface portions. Through the simultaneous application of these two additional measures, values of the quality factor and the critical magnetic flux density of about the same magnitude can always be obtained, since it is presumed that the germination of the Nb.sub.3 Sn layer occurs more homogeneously if the tin supply at the niobium surface is large, a tin source which is at a higher temperature than the niobium surface leads to a larger tin supply. On the other hand, an oxide layer initially prevents direct interaction of the tin with the metallic niobium. At temperatures of 600.degree. C., however, the oxygen is absorbed by the niobium material, and a thin tin film then comes into contact with the metallic niobium. This leads to a dense, uniform Nb.sub.3 Sn layer, since it has been determined that the anodized surface portions in partially anodized niobium samples are coated with a uniform Nb.sub.3 Sn layer, while the surface portions which are not anodized are only coated incompletely.
However, it is difficult to apply the additional measures mentioned if the geometrical shape of the surfaces to be coated is not favorable, since they can have only a small effect at so-called shaded places and nonuniform coating with the disturbances mentioned can develop there, as before. This danger is particularly great, for instance in cavity resonators of the TM.sub.010 type.